Noninvasive monitoring of maximalexpiratory and inspiratory flows(MIF and MEFrespectively)by using an electrical impedance-tomography(EIT)could facilitatethe early detection of changes inrespiration’s mechanical parametersduetothe new condition orresponse totreatments.We sought to confirmEIT-basedmeasuresofMIFas well asMEF against spirometryfor intubatedhypoxemic patients under controlled breathingas well as spontaneous breathing.Additionally, the regional distribution ofmaximum airflows could interact withlungpathology and increasethepossibility of further ventilationinjury.So, we also triedtoexamine the effectofsettings for mechanical airflow onregions ofMIFas well asMEF.
We performed a new studyofdatain two prospective, randomizedcross-sectionalstudies.We included intubated patientsadmitted to theintensive care unit (ICU) withacute hypoxemic respiratory failure(AHRF)and acute respiratory stress syndrome(ARDS)under pressure supportventilation(PSV, n=10) andthe volume-controlled ventilation(VCV, n20).We determined MIF and MIFusing spirometry, EIT and6 different ventilation configurations which were:higherthan. lower supportinPSV and greatersupport vs. lowerpressure of positive end-expiratory(PEEP)duringbothVCV and PSV.The regional airflows were evaluated byEITin non-dependent and dependentlung regions, as well.
MIF and impedanztomographie measuredthroughEIT wereclosely correlated tothose measured by spirometry overany condition(rangefromR2 0.629-0.776 and R2 0.606-0.772respectively, p<0.05onall) and with acceptable clinicalboundaries of disagreement.Higher PEEP significantly enhancedhomogeneity in the regionalpatternof MIF and MEFwhen ventilation is controlled in volume,by increasing airflows in thedependent lung regions and loweringthose in non-dependent regions.
EITprovides accurate noninvasive monitoringofMIFandMEF.This study also suggeststhepossibilitythat EITcan help to determinePSV and PEEPsettingto increase the homogeneity and consistency ofspreading and deflating regional airflows.
Electric impedance tomography(EIT)can be described asanoninvasive, bedside, radiation-free,dynamic lung imaging technique. EITprovides intrathoracic maps oflung impedance changes referenced toan initial value(i.e.,an end-expiratory lung volume measured from athe previousbreath) every20-50 milliseconds1.The changes in intrathoracic impedance measuredbyEIT are linearlylinked tothe global and regional tidal volumes andsustained at higher positive end-expiratorypressure (PEEP) levels [22.This means thatEITyields noninvasive bedside continuousmeasurement oflung volumechange duringinspiration and exhalation.
Inspiratory and expiratory flows correspondto therate at whichthe lung’s volume as it changesovertime.When patients are intubatedthey aretypically measured usingSpirometers that are connectedin the ventilator’s circuit, prior totheendotracheal tube , or insidethe ventilator.Global maximum inspiratory as well asexpiratoryflow(MIF and MEF respectively)measured bystandard spirometry depend onphysical properties in the respiratory system(namely lung compliance, lung pressure andairway resistance) .Therefore, monitoring ofMIF andMEF canhelp to determineadjustments to the ventilation system(e.g. by determiningthelevel of positive pressure associatedwithbettermechanics)or to determinetheefficacy of pharmacologic treatments(e.g. the increase inMIFand/or MEF in response toan bronchodilator drug) .However, spirometry provides onlyglobal estimates of MIF andMEF. However, heterogeneous distributionofmodified lung mechanics is ahallmark of acute hypoxemic respiratoryfailing(AHRF)along with acute respiratory stress syndrome(ARDS) [5The spirometry method is not able to detect the heterogeneous distribution.Damage to the alveoli can causecollapse of lung units tightlyclose to normal-, partial- and over-inflated units which can result indifferencesof regionalMIFas well asMEF values.These imbalances could increasetherisk of ventilator-induced lung injury(VILI)through various mechanisms, and settingscreating more homogeneous regional flowcould decrease the risk. Externalspirometry can leadtoaltered patterns of breathing andinaccurate measurementsas well.Therefore, a non-invasivebedsidedynamic method to measureglobal and regional MIF andMEFvalues canmake a great contribution toknowingAHRF and ARDSpatient’s pathophysiology andto provide personalized treatment.
In the current study,following preliminary findings from ananimals, we set outtovalidate inintubatedAHRFandARDS patientswho are receivingcontrolledventilation andEIT-based spontaneous breathing measures ofglobal MIF and MEF compared tostandardspirometry.We also investigatedtheimpact of higher vs. lowerpressure levels inregionalflows;it is our beliefthat higherPEEPand lower pressure support mightresult in a more uniform distribution oflocalMIFandMEF.
Materials and methods
We performed a new analysis of data collected during two prospective randomized crossover studies: in the first (pressure support ventilation (PSV) study) , ten intubated patients recovering from ARDS , lightly sedated (RASS – 2/0), undergoing PSV and admitted to the intensive care unit (ICU) of the university-affiliated San Gerardo Hospital, Monza, Italy, were enrolled; and in the second (volume-controlled ventilation (VCV) study) , twenty intubated, deeply sedated and paralyzed patients with AHRF (i.e., PaO2/FiO2 <=300, PEEP >=5 cmH2O, acute onset, no cardiac failure) or ARDS admitted to the same ICU were enrolled. Theethical committee ofSan Gerardo Hospital, Monza, Italy, approved theresearch,in accordance with the informed consent givenaccording tolocalguidelines.Additional details onthecriteria for inclusion and exclusionforthe twostudies are includedina data supplement online(Additionaldocument1).
Demographic data collection
Wecollected data on sex, ages, Simplified Acute Physiology Score IIscores, etiology, diagnosis andseverityof ARDS days onmechanical ventilationprior to enrollment in the studyforeverypatient.The mortality rate in hospital was also recordedtoo.
EIT andmonitoring of ventilation
Ineach patient, an EIT-dedicatedbelt,consisting of 16 equallyspaced electrodes, was placedclose to the thorax area atthefifth or sixthintercostalspace and connected toa commercialEIT monitor (PulmoVista 500, Drager Medical GmbH, Lubeck, Germany).Throughout all study phases,EITdata were generated byuse of small alternationelectrical currents rotating aroundpatient’s thorax, continuously recordedat 20 Hz. The data were then savedfor offline analysis, as previouslydiscussed .Synchronized toEITtracer data or airway pressures andairflows generated byventurism wererecorded continuously.
Further details aboutthe two protocolsare availablein theonline data supplement(Additionaldocument1.).
InPSV, in thePSV study,the patients went throughthe followingrandomized steps every 20 min:
Low support for PEEP in clinical(PSV low)as compared to.more support atPEEP in the clinical setting(PSV high);
Clinical supportforvery low levels of pressure(PSV-PEEP low)vs.clinical support at higher PEEP(PSV-PEEP high).
TheVCV study,the following phaseswere executedin a randomized order for crossovers,each lasting20 mins:
Protection VCV in low-PEEP(VCV-PEEP low)in comparison to.VCV that is protective at clinicalPEEPand 5cmH 2O (VCV-PEEP high).
EIT andventilation data
In an offline study ofEITtracer results obtained inthefinal minutesof each phase(analysis oftenbreaths) and analyzed theglobal and regional(same-sizeof non-dependent and dependent regions in the lung) noninvasive airflowswaveform,as described in the past[88.Briefly, instantaneous global andregionalexpiratory and inspiratoryairflowswere assessed asvariationsof the global and regionalimpedances measured every 50 ms and multiplied by thevolume/tidal impedance ratio ofthestudy phase in question anddivided by 50milliseconds. EIT airflow data werethen transformed from mL/msecL/min (Fig. 1) and the maximumMIFs from EITs that were global and regionaland MEF (MIFglob MIFglob, MIFnondepand MIFdepMEFglob with MEFnon-dep, MEFglob andMEFdep according to) wereidentified . Thevalues were averaged overfive to 10 consecutivebreathingcycles.